1. Field of the Invention
The present invention relates to a semiconductor memory, and more specifically to a non-volatile semiconductor memory comprising a plurality of memory cells each of which includes a memory transistor having a floating gate, so that each memory cell can be electrically written and electrically erased.
2. Description of Related Art
Conventionally, writing of data into this type of semiconductor memory has been performed by applying a high voltage between a control gate and a drain of a selected memory transistor, so that carriers in the form of a hole are injected into a floating gate of the selected memory transistor. A time required for completing the injection of the carriers into the floating gate is determined by a device structure including a thickness of gate insulator and others. In addition, as a means for reading a writing characteristics in which a threshold voltage will be brought into a negative due to movement of the carriers, there has been known a circuit for applying a negative potential to the control gate of the memory transistor.
In the above mentioned semiconductor memory, on the other hand, a plurality of memory cells have been arranged in the form of a matrix, and therefore, there has been provided a logic circuit for selecting one memory cell by applying an output of the logic circuit to a control gate of the memory transistor of the selected memory cell. However, since the control gate is electrically connected to a diffused region, it has been impossible to apply a negative potential to the control gate. For this reason, there has been no means for verifying a threshold voltage after written. Therefore, it is not possible to verify a data writing characteristics.
As mentioned above, since the conventional semiconductor memory cannot apply a negative voltage to the control gate, it is not possible to measure a threshold voltage after written. Because of this, the writing has been performed with a large margin. Namely, a writing time has been wastefully consumed. Therefore, the conventional semiconductor memory has a disadvantage in that a voluminous writing time has been required.
In addition, since it is not possible to directly measure the writing characteristics, a quality guarantee cannot be given on a change in time of the threshold voltage after written.